Interpretive Summary: Fermentation inhibitors generated from the low-cost lignocellulosic biomass conversion process poses a significant challenge for a sustainable lignocellulose-to-ethanol conversion industry. Development of tolerant ethanologenic yeast that are able to in situ detoxify aldehyde inhibitors have been demonstrated to be an efficient means to overcome the inhibitor stress. Knowledge on mechanisms of the tolerance and functional genes are limited. In this study, we report a novel aldehyde reductase gene isolated from ethanologenic yeast Saccharomyces cerevisiae NRRL Y-50049 that show strong reduction activities toward at least 14 aldehyde substrates among which, many are inhibitors generated from lignocellulosic biomass conversion hydrolysis. We characterized this gene that encodes an aldehyde reductase as a member of the ‘intermediate’ subfamily of short-chain dehydrogenase superfamily. Identification and understanding of this new type of enzyme impacts low-cost bioethanol production in the field of tolerant strain development and contributes basic knowledge on genome annotation of the yeast.

Technical Abstract:
Aldehyde inhibitors such as furfural, 5-hydroxymethylfurfural (HMF), anisaldehyde, benzaldehyde, cinnamaldehyde, and phenylaldehyde are commonly generated during lignocellulosic biomass conversion process for low-cost cellulosic ethanol production that interferes with subsequent microbial growth and fermentation. In situ detoxification of the aldehyde inhibitors is possible by the tolerant ethanologenic yeast that involves multiple genes including numerous functional reductases. In this study, we report a novel aldehyde reductase gene clone Y63 from ethanologenic yeast Saccharomyces cerevisiae NRRL Y12632, representing the uncharacterized ORF YGL157W, which demonstrated nicotinamide dinucleotide phosphate (NADPH)-dependent reduction activities toward at least 14 aldehyde substrates. The identity of gene clone Y63 is the same as YGL157W of SGD since a variation of only 35 nucleotides in genomic sequence and three amino acid residues were observed between the two that share the same length of 347 residues in size. As one of the highly induced genes, YGL157W of Y-12632 showed significantly high levels of transcript abundance in response to furfural and HMF challenges. Based on the deduced amino acid sequence and the most conserved functional motif analyses including closely related reductases from five other yeast species to this date, YGL157W was identified as a member of the subclass ‘intermediate’ of the SDR (short-chain dehydrogenase/reductase) superfamily with the following typical characteristics: the most conserved catalytic site to lie at Tyr169-X-X-X-Lys173; an indispensable reduction catalytic triad at Ser131, Tyr169, and Lys173; and an approved cofactor binding motif at Gly11-X-X-Gly14-X-X-Ala17 near the N-terminus. YGL039W, YDR541C, and YOL151W (GRE2) appeared to be the similar types of enzymes falling into the same category as the intermediate subfamily.